In recent years, hydraulic fracturing applications from boreholes in geological formations have expanded dramatically to meet the needs of such emerging technologies as in situ leaching, horizontal completion of oil and gas wells, methane gas mining, and non-explosive rock demolition. Years ago, hydraulic fracturing was characterized by generation of randomly oriented fractures and mere propping or extending of existing cracks or partings. Increasingly, success of a hydraulic fracturing job is dependent upon control of hydraulic fracture origin and orientation. In some applications, hydraulic fractures must originate at a specified location along the length of a borehole. In other applications, hydraulic fractures must run with a specified orientation to the local geologic structure, the borehole from which they originate, or some other structure.
The key to directional hydraulic fracturing is to restrict pressurized fluids and their egress to the desired fracturing plane so that tensile stresses are concentrated in the desired fracturing plane and the tensile strength of the geologic formation is exceeded in only the desired fracturing plane.
The prior art of borehole sealing is adept at isolating a particular section of borehole and preventing leakage of fracturing fluids into unconfined sections of a borehole, but this prior art is poorly suited to the task of directional hydraulic fracturing. Because their sealing elements can not be closely positioned, current borehole tools employing MECHANICAL PACKERS and INFLATABLE PACKERS can not restrict pressurized fluids to a desired fracturing plane. Because boreholes dilate in response to internal pressurization, an annular gap may develop between CEMENT PLUGS and the surrounding formation during borehole pressurization activities. Methods that use a PENETRATING FLUID placed at a selected elevation in a borehole surrounded above and below by a non-penetrating fluid to restrict fluid egress from the borehole do not concentrate tensile stresses in a plane.
The prior art of directional hydraulic fracturing typically resorts to BOREHOLE ALTERATION TECHNIQUES to control the subsequent direction of hydraulic fracturing. Borehole alteration techniques are used at the borehole interface with the expectation that subsequent hydraulic fracturing will follow in the orientation prescribed by the borehole alteration. Notching and fracturing techniques are most common whereas a subsequent hydraulic fracture is intended to follow in the orientation of a notch or fracture. WATERJET NOTCHING techniques use a waterjet to cut a notch into the borehole wall. The complexity of waterjet notching is largely dependent upon the geologic formation. Hard formations require such specialized waterjet cutting practices as ultra-high-pressure water jetting or abrasive water jetting. Soft formations sometimes require that a plastic casing patch be applied to the borehole before waterjet notching to prevent excessive borehole erosion. SHAPED EXPLOSIVE CHARGE techniques use explosives bundled in a designed configuration to notch a borehole. In weak geologic formations, a borehole patch is required to restrict the effects of the explosives to only the desired orientation. MECHANICAL NOTCHING techniques commonly use an indentor to generate a notch in a borehole wall. Unfortunately, these borehole alteration techniques are complicated operations involving special skills and tools.
Furthermore, borehole properties such as diameter, length, and wall conditions can make hydraulic fracturing difficult to control and can compromise the performance of current directional fracturing techniques. Short boreholes and boreholes with small diameter may exceed the dimensional requirements of some hydraulic fracturing hardware. Rough or uneven borehole walls may exceed the working limitations of some hydraulic fracturing techniques and corresponding equipment. To enable adequate borehole pressurization, the hydraulic fracturing technique must restrict permeation of the fracturing fluid to a rate commensurate with pumping equipment and the pressure required for fracturing.
Hydraulic fracturing applications are often cost sensitive. In some applications, hydraulic fracturing is uneconomical because of equipment expense or the purchase of expensive hydraulic equipment can be justified only on the basis of equipment recovery and re-use. When an unstable rock mass does not permit equipment recovery due to borehole closure, equipment cost can compromise the suitability of current directional fracturing techniques.